The present invention relates to semiconductor chip packaging. More particularly, the present invention relates to an improved compliant semiconductor package structure and methods for making the same.
The present invention relates to semiconductor chip packaging. More particularly, the present invention relates to an improved compliant semiconductor package structure and methods for making the same.
Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require many hundreds of connections to external devices.
Semiconductor chips commonly have been connected to electrical traces on mounting substrates by one of three methods: wire bonding, tape automated bonding, and flip-chip bonding. In wire bonding, the chip is positioned on a substrate with a bottom or back surface of the chip abutting the substrate and with the contact-bearing front or top surface of the chip facing upwardly, away from the substrate. Individual gold or aluminum wires are connected between the contacts on the chip and pads on the substrate. In tape automated bonding a flexible dielectric tape with a prefabricated array of leads thereon is positioned over the chip and substrate and the individual leads are bonded to the contacts on the chip and to pads on the substrate. In both wire bonding and conventional tape automated bonding, the pads on the substrate are arranged outside of the area covered by the chip, so that the wires or leads fan out from the chip to the surrounding pads. The area covered by the subassembly as a whole is considerably larger than the area covered by the chip. This makes the entire assembly substantially larger than it otherwise would be. Because the speed with which a microelectronic assembly can operate is inversely related to its size, this presents a serious drawback. Moreover, the wire bonding and tape automated bonding approaches are generally most workable with chips having contacts disposed in rows extending along the periphery of the chip. They generally do not lend themselves to use with chips having contacts disposed in a so-called area array, i.e., a grid-like pattern covering all or a substantial portion of the chip front surface.
In the flip-chip mounting technique, the contact bearing surface of the chip faces towards the substrate. Each contact on the chip is joined by a solder bond to the corresponding pad on the substrate, as by positioning solder balls on the substrate or chip, juxtaposing the chip with the substrate in the front-face-down orientation and momentarily melting or reflowing the solder. The flip-chip technique yields a compact assembly, which occupies an area of the substrate no larger than the area of the chip itself. However, flip-chip assemblies suffer from significant problems with thermal stress. The solder bonds between the chip contacts and substrate are substantially rigid. Changes in the size of the chip and of the substrate due to thermal expansion and contraction in service create substantial stresses in these rigid bonds, which in turn can lead to fatigue failure of the bonds. Moreover, it is difficult to test the chip before attaching it to the substrate, and hence difficult to maintain the required outgoing quality level in the finished assembly, particularly where the assembly includes numerous chips.
Numerous attempts have been made to solve the foregoing problem. Useful solutions are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266. Preferred embodiments of the structures disclosed in these patents incorporate flexible, sheet-like structures referred to as xe2x80x9cinterposersxe2x80x9d or xe2x80x9cchip carriersxe2x80x9d. The preferred chip carriers have a plurality of terminals disposed on a flexible, sheet-like top layer. In use, the interposer is disposed on the front or contact bearing surface of the chip with the terminals facing upwardly, away from the chip. The terminals are then connected to the contacts of the chip. Most preferably, this connection is made by bonding prefabricated leads on the interposer to the chip contacts, using a tool engaged with the lead. The completed assembly is then connected to a substrate, as by bonding the terminals of the chip carrier to the substrate. Because the leads and the dielectric layer of the chip carrier are flexible, the terminals on the chip carrier can move relative to the contacts on the chip without imposing significant stresses on the bonds between the leads and the chip, or on the bonds between the terminals and the substrate. Thus, the assembly can compensate for thermal effects. Moreover, the assembly most preferably includes a compliant layer disposed between the terminals on the chip carrier and the face of the chip itself as, for example, an elastomeric layer incorporated in the chip carrier and disposed between the dielectric layer of the chip carrier and the chip. Such a compliant structure permits displacement of the individual terminals independently towards the chip. This permits effective engagement between the subassembly and a test fixture. Thus, a test fixture incorporating numerous electrical contacts can be engaged with all of the terminals in the subassembly despite minor variations in the height of the terminals. The subassembly can be tested before it is bonded to a substrate so as to provide a tested, known, good part to the substrate assembly operation. This in turn provides very substantial economic and quality advantages.
Commonly owned U.S. Pat. No. 5,455,390 describes a further improvement. Components according to preferred embodiments of the ""390 patent use a flexible, dielectric top sheet having top and bottom surfaces. A plurality of terminals are mounted on the top sheet. A support layer is disposed underneath the top sheet, the support layer having a bottom surface remote from the top sheet. A plurality of electrically conductive, elongated leads are connected to the terminals on the top sheet and extend generally side by side downwardly from the terminals through the support layer. Each lead has a lower end at the bottom surface of the support layer. The lower ends of the leads have conductive bonding materials as, for example, eutectic bonding metals. The support layer surrounds and supports the leads.
Components of this type can be connected to microelectronic elements such as semiconductor chips or wafers by juxtaposing the bottom surface of the support layer with the contact-bearing surface of the chip so as to bring the lower ends of the leads into engagement with the contacts on the chip, and then subjecting the assembly to elevated temperature and pressure conditions. All of the lower ends of the leads bond to the contacts on the chip substantially simultaneously. The bonded leads connect the terminals of the top sheet with the contacts on the chip. The support layer desirably is either formed from a relatively low-modulus, compliant material, or else is removed and replaced after the lead bonding step with such a compliant material. In the finished assembly, the terminals desirably are movable with respect to the chip to permit testing and to compensate for thermal effects. However, the components and methods of the ""390 patent provide further advantages, including the ability to make all of the bonds to the chip or other component in a single lamination-like process step. The components and methods of the ""390 application are especially advantageous when used with chips or other microelectronic elements having contacts disposed in an area array.
Despite the positive results of the aforementioned commonly owned inventions, still further improvements would be desirable.
The present invention contemplates a method of creating a compliant semiconductor chip package assembly and the semiconductor chip package assembly created therefrom.
In a fabrication process according to one aspect of the invention, a first dielectric protective layer is provided on a contact bearing surface of a semiconductor chip. The semiconductor chip has a central region bounded by the chip contacts and a set of apertures. The apertures in the dielectric protective layer are provided such that the chip contacts are exposed. This first dielectric protective layer may actually be the silicon dioxide passivation layer of the semiconductor chip.
Second, a compliant layer, preferably consisting of silicone, flexibilized epoxy, a thermosetting polymer or polyimide is provided atop the first dielectric protective layer is provided within the central region. The compliant layer is formed such that it has a substantially flat top surface and edges that gradually slope down to the top surface of the first dielectric protective layer. The sloping edges of the compliant layer may be manufactured to have a first transition region near the top surface of the compliant layer and a second transition region near the bottom surface of the compliant layer such that both the first transition region and the second transition region have a radius of curvature.
Finally, bond ribbons are selectively formed atop both the first dielectric protective layer and the compliant layer such that each bond ribbon electrically connects each chip contact to a respective terminal position on the compliant layer. The bond ribbons may be selectively formed using a variety of techniques, such as by electroplating or by electroless plating followed by selective etching. The terminal positions are the conductive elements that connect the finished assembly to a separate substrate, e.g. a printed circuit board.
The method described above may further include the step of providing for a second dielectric protective layer atop the bond ribbons and the compliant layer after the bond ribbon electroplating step is performed. This optional second dielectric protective layer is fabricated with a set of apertures that expose the underlying terminal positions on the compliant layer.
Additionally, the method described above may further include the optional step of providing for an encapsulant layer above the bond ribbons. If this optional step is performed, it is performed after the step of selectively electroplating the bond ribbons. Like the first dielectric layer, the encapsulant layer is fabricated with a set of apertures so that the terminal positions are exposed. The encapsulant layer material consists preferably of either a curable liquid, such as silicone, a flexibilized epoxy or a gel. This optional step may also be performed just prior to the optional step of providing for a second dielectric protective layer.
In another aspect of the invention, a method of making a compliant microelectronic assembly includes providing a microelectronic element, such as a semiconductor chip, having a first surface and a plurality of contacts disposed on the first surface thereof and forming a compliant layer over the first surface of the microelectronic element. The compliant layer typically has a bottom surface facing toward the first surface of the microelectronic element, a top surface facing upwardly away from the microelectronic element and one or more edge surfaces extending between the top and bottom surfaces. The edge surfaces of the compliant layer are preferably sloping surfaces which extend in both vertical and horizontal directions. At least some of the sloping edge surfaces preferably have first transition regions near the top surface of the compliant layer and second transition regions near the bottom surface of the compliant layer, the first and second transition regions having respective radii of curvature.
In certain embodiments, before the compliant layer is formed, a first dielectric protective layer, such as a layer including a silicon dioxide passivation layer, may be provided on the first surface of the microelectronic element. The first dielectric protective layer may have a plurality of apertures therein so that the contacts are accessible therethrough. The compliant layer described above can then be provided over the dielectric protective layer.
Bond ribbons may then be selectively formed over the compliant layer. The bond ribbons preferably extend over both the top surface of the compliant layer and one or more edge surfaces of the compliant layer. The bond ribbons electrically connect the contacts to conductive terminals overlying the top surface of the compliant layer. Before the bond ribbons are formed, a barrier metal layer may be deposited over the contacts so as to prevent undesired chemical reactions between the contacts and the bond ribbons. In one embodiment, the bond ribbons are formed by selectively electroplating the bond ribbons atop the first dielectric protective layer and the compliant layer. After the bond ribbons have been formed, a dielectric cover layer may be formed over the compliant layer and the bond ribbons. The dielectric cover layer may have a plurality of apertures therein so that the terminals are accessible therethrough. In other embodiments, an encapsulant layer may be provided atop the exposed surfaces of the bond ribbons. The encapsulant layer is generally a material selected from the group consisting of silicone, flexibilized epoxy, thermoplastic and gel. Next, a second dielectric protective layer or cover layer may be provided over the encapsulant layer. The second dielectric protective layer also preferably has a plurality of apertures therein so that the terminals are accessible therethrough.
The compliant layer may include one or more apertures therein so that the contacts are accessible through the apertures. The one or more apertures may include one or more groups of apertures encompassing a plurality of the contacts. The edge surfaces of the compliant layer may include one or more aperture edge surfaces bounding the apertures, with at least some of the bond ribbons being formed over the aperture edge surfaces. The compliant layer may be formed by engaging the microelectronic element with a mold so that one or more projections on the mold contact the first surface of the microelectronic element. A flowable composition may be introduced around the projections and the flowable composition set to provide a compliant layer. The microelectronic layer may then be separated from the mold. The one or more apertures are typically formed in the space occupied by the projections.
In certain embodiments, the contacts on the microelectronic element are disposed in an area array, and the one or more apertures in the compliant layer include a plurality of apertures disposed in an array corresponding to the array of contacts so that each contact is encompassed in a respective aperture. In other embodiments, the contacts on the microelectronic element may be disposed in a first region of the first surface, with the compliant layer overlying a second region of the first surface, and one or more edge surfaces including one or more border edge surfaces extending along one or more borders between the first and second regions. In still other embodiments, the contacts on the microelectronic element are disposed in a central region of the first surface and the compliant layer overlies a peripheral region of the first surface.
In another embodiment, a method of making a compliant microelectronic package includes providing a supporting element having an upwardly-facing top surface and juxtaposing a microelectronic element including a first surface having a plurality of contacts thereon with the supporting element so that the first surface of the microelectronic element is disposed alongside the top surface of the supporting element. The first surface of the microelectronic element and the top surface of the supporting element may be substantially coplanar after the juxtaposing step.
A compliant layer may then be provided over the top surface of the supporting element, the compliant layer having a top surface remote from the top surface of the supporting element, a bottom surface and an edge surface extending between the top surface and the bottom surface. In certain embodiments, a portion of the compliant layer extends over the first surface of the microelectronic element, with one or more edge surfaces of the compliant layer overlying the first surface of the microelectronic element. Bond ribbons may then be selectively formed atop the compliant layer, the bond ribbons electrically interconnecting the contacts of the microelectronic element with conductive terminals overlying the top surface of the compliant layer.
The supporting structure described above may have a central aperture therein so that the microelectronic element may be placed in the central aperture after being juxtaposed with the supporting element. After the juxtaposing step, the first surface of the microelectronic element and the top surface of the supporting structure are preferably substantially coplanar.
In certain embodiments, the compliant chip assembly may include a ground plane electrically interconnected with at least one of the bond ribbons. The ground plane may include a plurality of apertures therein so that the terminals are accessible through the apertures.
The methods described above can be applied simultaneously to a multiplicity of undiced semiconductor chips on a wafer to form a corresponding multiplicity of compliant semiconductor chip packages. After the bond ribbons have been formed on the packages, individual packages may be severed or diced from the wafer to provide separate and distinct chip packages. The methods may also be applied to a multiplicity of adjacent semiconductor chips arranged in an array to form a corresponding multiplicity of compliant semiconductor chip packages, whereby the packages are diced after the bond ribbons have been formed.
A further aspect of the present invention includes the structure of a unique compliant semiconductor chip package having fan-in type leads. The compliant semiconductor chip package is comprised of (1) a semiconductor chip having a plurality of peripheral bonding pads on a face surface thereof and a central region bound by the peripheral bonding pads; (2) a first dielectric protective layer having a first surface, a second surface and apertures, wherein the first surface of the first dielectric layer is joined to the face surface of the semiconductor chip and the peripheral bonding pads are exposed through the apertures; (3) a compliant layer having a top surface and a bottom surface, wherein the bottom surface of the compliant layer is joined to the second surface of the first dielectric layer within the central region of the semiconductor chip package; and (4) a plurality of electrically conductive bond ribbons, each bond ribbon having a first end that electrically couples to a respective peripheral bonding pad of the semiconductor chip and a second end that joins to the top surface of the compliant layer to form a package terminal.
The package terminals of the completed package are configured in an array that has an area smaller than the area bound by the peripheral bonding pads on the face of the semiconductor chip. In other words, the package has fan-in leads that permits minimization of the overall package size.
For increased reliability, the compliant layer has sloped peripheral edges so that the overlying bond ribbons are curved rather than kinked.
The compliant semiconductor chip package may also have a compliant layer characterized by an array of bumped protrusions. The bumped protrusions support the overlying conductive terminal position ends of the bond ribbons and function as conductive balls that join to a substrate thus forming a ball grid array type interconnection. Alternate to the bumped protrusions, the compliant layer may have an array of concavities that are useful for placement of solder balls into each concavity. This arrangement is also useful for a ball grid array type interconnect.